1. Field of the Invention
The present invention relates to a control of car power brakes and more particularly to antiskid control for decreasing wheel skidding on a road surface by damping the power of the brakes to permit acceleration of the rotation of wheels, where the wheels are decelerated abruptly by the brake power, causing heavy skidding on the road surface.
2. Description of the Prior Art
If a car is braked while running on a wet or frozen road, the wheels stop suddenly and slip on the road surface thus, the car is capable of skidding on the road surface, which may often take place when the car runs at high speed or is braked suddenly. If the wheels are locked while the car runs forward, steering gets difficult and causes car accidents.
Now, therefore, acceleration is computed by detecting wheel rotational speed, a slip factor is computed as:
Sp=1--wheel rotational speed/car speed and thus when the slip factor in the brake equals a predetermined value Sh or over, the brake pressure is stepped down to recover the wheel rotation. The car speed cannot be detected accurately when slipping. Consequently, a reference speed regarded as the car speed is obtained through various arithmetic operations according to the wheel rotational speed. In the case of four-wheel cars, for example, a top wheel speed is specified as the reference speed.
Such a type of antiskid control is disclosed, for example, in Japanese Patent Laid-Open No. 54789/1974. In the antiskid control disclosed therein, a "reducing" domain and an "intensifying" domain are so divided as shown in FIG. 6, and where the slip factor equals 20% to 60%, the wheel brake pressure is reduced at a wheel acceleration (plus indicating acceleration, and minus indicating deceleration) being less than G, and is intensified at an acceleration of G or over. When the slip factor is less than 20%, a boundary value is lowered to G-.alpha., and when exceeding 60%, the boundary value is raised to G+.beta..
For setting the wheel brake pressure, more appropriately, it is preferable that a range (0 to 100%) of the slip factor be further subdivided to specify the boundary value accurately, and for smoothing a fluctuation of the wheel brake pressure, it is preferable that the brake pressure be divided into three domains by interposing a "hold" domain between those "reducing" and "intensifying" or further divided into five domains by providing domains "slow reducing", "hold" and "slow intensifying" further to the two stages of "reducing" and "intensifying".
However, as the slip factor is subdivided, or, for example, the boundary value coordinating with the slip factor is specified analogically, the memory capacity becomes large. A boundary group increases by one at every increase in one of the domain segments, therefore according as the slip factor is subdivided, the domain segment is subdivided accordingly, and the boundary values increase immensely in number, and thus the memory capacity therefor becomes immensely large. Further, when the domain segment is modified to a car deceleration (3 segments) like the embodiment (FIG. 2a to FIG. 2c) of the invention which will be described hereinafter, the number of boundary values is further multiple three (segment number of the deceleration) times, and the necessary memory capacity gets immensely large, accordingly. If the required memory capacity increases, then the number of digits of the address data for memory reading is multiplied, the number of signal lines of a microcomputer system used for antiskid control increases, and the system increases in scale accordingly; and the hardware and a controlling logic becomes complicated.